Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP⁻ (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.     

Femtosecond dynamics of electron transfer in a neutral organic mixed-valence compound

In this article we report a femtosecond time-resolved transient absorption study of a neutral organic mixed-valence (MV) compound with the aim to gain insight into its charge-transfer dynamics upon optical excitation. The back-electron transfer was investigated in five different solvents, toluene, dibutyl ether, methyl-tert-butyl ether (MTBE), benzonitrile and n-hexane. In the pump step, the molecule was excited at 760 nm and 850 nm into the intervalence charge-transfer band. The resulting transients can be described by two time constant. We assign one time constant to the rearrangement of solvent molecules in the charge-transfer state and the second time constant to back-electron transfer to the electronic ground state. Back-electron transfer rates range from 1.5 × 10¹² s⁻¹ in benzonitrile through 8.3 × 10¹¹ s⁻¹ in MTBE, around 1.6 × 10¹¹ s⁻¹ in dibutylether and toluene and to 3.8 × 10⁹ s⁻¹ in n-hexane.     

混合价化合物的研究(Ⅱ)─—电化学及现场光谱电化学方法研究非质子介质中Dawson结构杂多阴离子[HAs_2Mo_(18)O_(62)]~(n-)的电还原性质

以电化学和现场紫外－可见－近红外及现场FTIR光谱电化学方法对Dawson结构杂多阴离子［HAS2Mo18O62］5－在非质子介质（CH3CN）中的电还原过程进行了研究．结果表明，该杂多阴离子在非质子介质中经历4步单电子还原反应，所产生的杂多蓝阴离子在近红外区出现宽广的价间电荷转移吸收带，而红外区Mo＝O端键及Mo—O—Mo桥键的特征吸收峰在还原后均有不同程度的红移．     

Synthesis and characterization of tetrahedrally and octahedrally coordinated mixed-valence Co(II)/Co(III) complex with thiosemicarbazone-based ligand

Synthesis, characterization, and X-ray crystal structure for a mixed-valence binuclear Co(II)/Co(III) complex with the dianionic dithiolate form of a pentadentate ligand 2,6-diacetylpyridinebis(thiosemicarbazone) are reported. A new synthetic methodology has been employed replacing usual cobalt(II) salts by [Co(NH₃)₅Cl]Cl₂ as a precursor. The coordination geometry of cobalt(II), CoN₂S₂, was found to be distorted tetrahedral, whereas the cobalt(III) coordination sphere, CoN₄S₂, is slightly distorted octahedral. The magnetic behavior and molar conductivity of the complex are in agreement with the mixed-valence state.     

A novel extended architecture with 4·6 topology based on mixed-valence Wells-Dawson arsenotungstate and mixed-ligand Cu(I) units

A novel inorganic-organic hybrid compound based on mixed-valence Wells-Dawson arsenotungstate and mixed-ligand Cu(I) units, Cu₈^I(imi)₄(bpy)₆(H₂O)[As₂^VW₂^VW₁₆^VlO₆₂]·2H₂O (1) (bpy=4,4′-bipydine; imi=imidazole), has been hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, thermal gravimetric analysis, luminescent spectrum and single crystal X-ray diffraction. Single-crystal X-ray diffraction revealed that four terminal and three bridging oxygen atoms of the Wells-Dawson cluster are coordinated to Cu(I) ions and form an unprecedented hepta-supporting polyoxometalate. The functionalized arsenotungstates are further connected by two kinds of tridentate linkers, Imi-Cu-(bpy)-Cu-(bpy)-Cu-(bpy)-Cu-Imi and Imi-Cu-(bpy)-Cu-(bpy)-Cu-H₂O, to construct a 3D framework with 4⁶·6⁴ topology. The hybrid material has an intense emission at about 397 nm.     

密度泛函理论在分子磁学中的应用——混合价三核锰配合物磁性的理论研究

在对混合价化合物分类的基础上, 将双核耦合模型推广到三核磁耦合体系, 重点研究定域与离域两类混合价化合物的磁学性质. 以混合价三核锰为例, 应用密度泛函理论方法计算了定域与离域 [Mn3O(O2CH)6L3]z+(L=pyridine; z=0)化合物的电子结构, 得到与实验可比较的磁耦合常数J. 结果表明, 密度泛函理论(DFT)结合对称性破损(BS)方法可用于此三核混合价体系; 对于完全离域的混合价Mn3Ⅱ,Ⅲ,Ⅲ体系, 必须考虑电子的离域作用对磁耦合的影响, 其Hamiltonian量应该包含双交换参数B. 为便于比较, 同时计算了等价三核锰化合物[Mn3O(O2CH)6L3]z+(L=pyridine; z=1)的磁学性质.     

High-dimensional mixed-valence copper cyanide complexes: Syntheses,structural characterizations and magnetism

Reactions of CuCl₂ with different CN complexes in presence of a neutral ancillary ligand lead to two novel mixed-valence Cu complexes [Cu^II(bpy)Cu^I(CN)₃]_n, 1 (bpy = 2,2′-bipyridine) and {[Cu^II(tn)₂][Cu^I₄(CN)₆]}_n2 (tn = 1,3-diaminopropane). For compound 1, the asymmetric unit involves two Cu ions Cu1 and Cu2 (Cu^I and Cu^II centres, respectively) which strongly differ in their environments. The Cu1 ion presents a CuC₄ pseudo-tetrahedral geometry, while the Cu2 ion presents a CuN₅ slightly distorted square-pyramidal geometry. The extended structure of 1 is generated by three cyano ligands which differ in their coordination modes. One CN group has a μ₃ coordination mode and bridges two Cu^I and one Cu^II ion, while the two other CN groups act as μ₂ bridges leading to a sophisticated 3-D structure. As for 1, the asymmetric unit of 2 involves three crystallographically different Cu ions (Cu1A and Cu1B, presumably Cu^I centres, and Cu2 presumably Cu^II centres). The Cu2 ion presents centrosymmetric CuN₄ coordination environments involving four nitrogen atoms from two bidentate tn ligands; while the Cu1A and Cu1B ions are three coordinated to cyano groups. The structure can be described as formed by 18-membered “[Cu^I(CN)]₆” planar metallocycles that are connected to their six neighbors to generate 2-D sheets; these sheets stack forming infinite hexagonal channels in which the [Cu(tn)₂]²⁺ units are located. Magnetic measurements show an unexpected weak ferromagnetic coupling (θ = 0.239(1) K) of the Cu^II ions through the long and “a priori diamagnetic” –NC–Cu^I–CN– bridges in compound 1 and an essentially paramagnetic behavior in compound 2.     

New mixed-valence chromium structure type: NH4Cr(CrO4)2

Synthesis and crystal structure of a new structure type of mixed Cr(III)/Cr(VI) chromates is reported. NH₄Cr(CrO₄)₂ was prepared from CrO₃ in the presence of (NH₄)₂Ce(NO₃)₆. Since this is the first preparation of mixed valence ternary chromium oxides from aqueous solution, a reaction pathway for this synthesis is suggested. The crystal structure of NH₄Cr(CrO₄)₂ has been determined from three-dimensional X-ray data collected at low temperature, 173 K. The structure belongs to the orthorhombic space group Pnma, with a=14.5206(10), b=5.4826(4), and Z=4. The title compound consists of corner-sharing chromium(III) octahedra and chromium(VI) tetrahedra forming a three-dimensional network with the composition [Cr(CrO₄)₂]_n^n-, containing channels in which zigzag rows of ammonium ions balance the net charge.